In optical telecommunication systems, one of the many difficulties encountered is the chromatic dispersion of light signals propagating over long distances in optical media such as optical fibers. Chromatic dispersion causes light pulses to spread out as they travel along an optical fiber. It occurs because the different spectral components at different wavelengths in a pulse travel at slightly different speeds. An optical pulse, which comprises different optical spectral components, therefore, can be broadened or distorted in shape after propagation through a distance in such a dispersive optical medium. This dispersion effect can be undesirable and even adverse for certain applications such as optical communication systems where information is encoded, processed, and transmitted through optical pulses. As the pulses spread, they can overlap and interfere with each other, thereby impacting signal integrity and limiting the transmission bit rate, the transmission bandwidth, and other performance factors of the optical communication systems. The effect becomes more pronounced at higher data rates. Pulses at different wavelengths typically suffer different amounts of dispersion. The chromatic dispersion in standard single-mode optical fiber is nominally 17 ps/(nm·km) in the 1550 nm telecommunication window, but this value changes as a function of the wavelength: its value changes by about 2 ps/(nm·km) between 1530 nm and 1565 nm.
Correction Methods
One way to mitigate the chromatic dispersion in dispersive optical fibers and other optical transmission media is to recompress the optical pulses using an optical element that provides dispersion that is just the opposite of the one of the fiber link. This process is referred to as dispersion compensation.
A dispersion compensating fiber (DCF) is a specialty optical fiber used to compensate for the dispersive effects encountered during signal transmission. Basically, this fiber has a dispersion characteristic of opposite sign to the optical fiber used for transmission. While a dispersion compensating fiber is generally a broadband solution to first order dispersion (dispersion slope), it does not properly compensate for second order dispersion. That is, the optimum length of these specialty fibers varies with channel wavelength. Thus, in a WDM system where multiple wavelengths are transmitted, no one length of dispersion compensating fiber precisely accommodates all channel wavelengths.
An alternative way of providing dispersion is based on Fiber Bragg gratings (FBGs), a well-established technology for optical telecommunications. Basically, a Bragg grating allows light propagating into an optical fiber to be reflected back when its wavelength corresponds to the grating's Bragg wavelength, related to its period. A chirped FBG, in which the grating period varies along the fiber axis, represents a well-known solution for compensating the chromatic dispersion of an optical fiber link (F. Ouellette, “Dispersion Cancellation Using Linearly Chirped Bragg Grating Filters in Optical Waveguides”, Opt. Lett., 12, pp. 847-849, 1987). Such a grating compensates for the accumulated dispersion since the group delay varies as a function of the wavelength. An appropriate grating can be fabricated such that the wavelength dependence of its group delay is just the opposite of that of the fiber link.
Depending on system network configuration, different levels of dispersion correction may be required, and often to compensate residual dispersion error of a DCF spool a zero-centered of dispersion range is needed for compensating the residual dispersion error remaining. By themselves, single chirped FBGs are not well suited for obtaining such zero centered dispersion levels. There is therefore a need for a device offering this feature. There is also a need for a device allowing an increase of the dispersion range.
Critical factors that affect dispersion compensation at high bit rate are changing traffic patterns, temperature fluctuations along the fiber, modulation format, component dispersion levels and dispersion variations in the transmission fiber (from manufacturing variances). To accommodate these factors, 40 Gb/s systems require not only fixed, broadband slope-compensated dispersion-compensating devices, but also tunable dispersion technology to adjust the dispersion compensation in real-time for different WDM channels.
Prior Art
The wavelength of peak reflection for a Bragg grating can be shifted by a change in either the strain or the temperature (or both) imposed on the grating. If the grating is subject to a strain or temperature gradient the modulation period of the index of refraction and the mean index of refraction becomes a function of position along the grating.
It is known in the art how to tune FBGs for various purposes, among which methods for creating tunable dispersion compensators.
If a linearly chirped FBG is uniformly stretched, the period is changed, and accordingly the Bragg reflection wavelength is also changed, but the dispersion remains unchanged. A similar situation pertains if, instead of stretching the fiber, a uniform heating is applied to the grating.
On the other hand, a non uniform heating, such as to produce a thermal gradient along the waveguide axis in the region of the grating, induces a chirp in the grating, or modifies an existing one. Controlling the magnitude of the thermal gradient controls the magnitude of the resulting chirp, and thus there is provided a form of adjustable amplitude linear dispersion compensation device. Such a device is for instance described by different implementations described hereinafter.
U.S. Pat. No. 5,671,307 (LAUZON et al.) discloses the use of a temperature gradient to impose a chirp on a FBG. By inducing a uniform linear variation of the local temperature over the length of the FBG, a slope variation of the time delay can be obtained, resulting in a variation of the dispersion compensation. The temperature gradient is realized by providing heat conductive means such as a thin brass plate to hold the portion of the fiber provided with the Bragg grating, and pairs of Peltier effect plates sandwiching each end of the fiber to selectively apply and dissipate heat to end from the ends of the fiber. Lauzon suggests that the device might be used as an accurately tunable dispersion compensator for optical fiber communication links. This pure thermal approach avoids any stresses in the fiber, allowing highly reliable implementations of the principle, as for the one given in the Canadian patent applications no. 2,371,106 and 2,383,807 (LACHANCE et al.) where a power efficient means for obtaining a linear temperature gradient in a thin conductive rod are disclosed.
Based on the same idea, European patent No. 0 997 764 (EGGLETON et al.) disclose an optical waveguide grating with adjustable chirp formed by a waveguide grating in thermal contact with an electrically controllable heat-transducing body which varies the temperature along the length of the grating. The heat transducing body, formed for example by a tapered film coating whose resistance varies along the length of the grating, can generate heat on the fiber to establish a temperature gradient along the grating.
A plurality of localized heaters can also be used along the length of the chirped FBG to alter its properties in order to tune the chirp and to produce tunable dispersion compensators. U.S. Patent Application 2002/048430 (HASHIMOTO) presents such an approach where an optical fiber is coupled to a succession of localized heaters mounted on a substrate. Linear temperature gradients are obtained which tune the dispersion in the linearly chirped FBG placed in close contact.
Similarly, if the waveguide is subjected to a stretching that is not uniform, but is such as to produce a strain gradient along the waveguide axis, then the effect is to produce a controllable amplitude of chirp. European patent No. 0 867 736 (FARRIES et al.) discloses a temperature-based device that combines the application of a temperature gradient and an optical strain to modify the optical properties of the grating. T. Imai et al. (“Dispersion Tuning of a Linearly Chirped Fiber Bragg Grating Without a Center Wavelength Shift by Applying a Strain Gradient”, IEEE June 1998, pp. 845-847) and U.S. Pat. No. 6,360,042 (LONG) describe devices in which a strain gradient is imparted to an optical fiber waveguide by bonding a portion of its length to a cantilever, and then bending that cantilever. U.S. Pat. No. 5,694,501 (ALAVIE) is another example of such a device in which a strain gradient is imparted to an optical fiber by cantilever bending and also by bonding it to the side of a stack of electrostrictive elements, and then applying a differential drive to those elements. The use of magnetostriction for grating chirping can also be used, as disclosed by U.S. Pat. No. 6,122,421 (ADAMS et al.). This patent discloses a programmable and latchable device for chromatic dispersion compensation based on a gradient magnetostrictive body bonded along the length of the fiber grating. In such a device, the magnetic field causes the body to expand or contract depending on the material. These devices however imply gluing the fiber to a metallic block along its entire length, which in practice is a technologically challenging operation.
The uniform stretching of an optical waveguide possessing a chirped Bragg grating with a quadratic component of its chirp can also induce a change in the linear dispersion afforded by the structure, as described in U.S. Pat. No. 5,982,963 (FENG). This approach allows a tuning of the dispersion but the spectral duty factor is limited to about 25%. Furthermore, this method relies on mechanical stretching which may cause fiber fatigue and degrade long-term reliability.
Another tunable dispersion compensator based on uniformly straining quadratically chirped FBGs is presented in U.S. Pat. No. 6,363,187 (FELLS) and in U.S. Pat. No. 6,381,388 (EPWORTH). In an effort to combat the transmission penalty associated with a quadratic chirp, this patent uses the reflection in a second Bragg grating identical to the first, but oriented to provide a quadratic component of chirp that has the opposite sign to that of the first Bragg reflection grating, and with a substantially matched modulus.
While providing useful art related to the tunability of Bragg gratings, none of the above-mentioned references discloses an adjustable, or tunable dispersion compensation device with appropriate ranges of dispersion correction. Such a device could be one designed for operation on its own for achieving the totality of dispersion compensation. Alternatively, it could be one designed for operation in association with a fixed amplitude dispersion compensation device, such as a length of DCF. The adjustable device may be operated with some form of feedback control loop to provide active compensation that can respond to dynamic changes of dispersion within the system, and in suitable circumstances to step changes resulting from re-routing occasioned for instance by a partial failure of the system such as a transmission fiber break.